This invention relates to electronic amplifiers, and more particularly to switched-mode radio frequency (RF) power amplifiers.
Transmitters in many modern communication systems, such as cellular radio systems having carrier frequencies of 1-2 gigahertz (GHz) or so, need to have wide bandwidth, wide dynamic range, and high accuracy (low distortion) in phase and envelope to deal with modern modulation schemes that enable effective use of allocated bandwidth. In addition, it is currently preferable that high-performance amplifiers be implemented in CMOS for reasons of cost and integration. Transmitters in battery-powered devices need to be efficient so that battery energy is conserved.
In conventional radio transmitters, the signal information is often represented as two channels in quadrature phase that can be mixed together to form a combined low-power signal that is amplified for transmission. A linear power amplifier is needed for proper amplification of the combined signal, but there is a trade-off between efficiency and linearity in RF power amplifiers. If high linearity is required, a Class A amplifier can be used, but at the cost of low efficiency. If a constant-envelope signal is to be amplified so that linearity is not critical, a high-efficiency switched-mode (Class D, E, or F) amplifier can be used. Class D amplifiers also can provide high power with low peaks in current and voltage, behavior that is important in CMOS implementations due to the limited breakdown voltages of CMOS devices.
To enable a Class D amplifier to handle signals with non-constant envelopes, the amplifier can use a form of pulse width modulator (PWM) for linearization, such as described in F. Raab, “Radio Frequency Pulsewidth Modulation”, IEEE Trans. Comm. pp. 958-966 (August 1973); M. Nielsen and T. Larsen, “An RF Pulse Width Modulator for Switch-Mode Power Amplification of Varying Envelope Signals”, Silicon Monolithic Integrated Circuits in RF Systems, pp. 277-280, Aalborg University (2007); and International Publication WO 2008/002225 A1 by H. Sjöland, for example.
A radio transmitter combining two or more outputs can use PWM in several different ways, but the basic concept used to pulse-width modulate an RF signal is much the same as for a low-frequency Class D amplifier employing PWM. One difference is that instead of low-pass filtering the output signal to extract information at the same frequency as the input signal to an amplifier, a band-pass filter (BPF) is used in a transmitter to extract information around the PWM switching frequency. This is sometimes called band-pass PWM or BP-PWM.
As described in WO 2008/002225, FIG. 1 is a block diagram of a portion of an RF transmitter that includes a switched-mode power amplifier 10, an output band-pass filter BPF, and an antenna 12. The amplifier 10 receives an input envelope signal input EI that is connected to a first input of an arithmetic subtractor SUB. The output of the subtracting unit SUB is provided to an amplifier Av, whose output is provided to a pulse-width modulator PWM that also receives an RF carrier signal C that is to be provided with phase-information content and transmitted. The output of the modulator PWM is provided to a power amplifier PA that receives a supply voltage Vdd and provides an amplified version of the output of the modulator PWM to the bandpass filter BPF, which is connected to the antenna 12. A second input of the subtractor SUB receives a feedback signal from the output of the power amplifier PA. The feedback signal is produced by a low-pass filter LPF that is connected to the output of the power amplifier PA. The output of the filter LPF is digitized by a first analog-to digital (A/D) converter A/D1 and provided to a digital signal processor DSP. The supply voltage Vdd is digitized by a second A/D converter A/D2 and provided to the processor DSP, which is suitably configured to produce the feedback signal that is converted from digital form to analog form by a D/A converter D/A and provided to the subtractor SUB.
As noted above, PWMs can be used in many ways with switched-mode amplifiers and signals having non-constant envelopes. FIG. 2 depicts a PWM 200 that can accurately produce two (differential) output signals, with the envelope (amplitude) information able to be used in a feedback loop using low-pass filters, such as shown in FIG. 1 and described in the Nielsen et al. publication cited above. The modulator 200 includes two comparators 202, 204 that produce output signals A, B, respectively, and an inverter 206 connected to the comparator 204. When connected as shown and provided with envelope and phase components of an input signal (i.e., an input signal presented in polar, rather than Cartesian, coordinates), the comparator output signals A, B are trains of pulses that have varying widths, such as those illustrated in FIG. 2. The difference signal A-B is also illustrated in FIG. 2.
Despite its accuracy, a PWM like that depicted in FIG. 2 does not use its output signals A, B efficiently. With a differential output, both positive and negative output voltages can be achieved, but as illustrated in FIG. 2, the difference signal A-B is monopolar. To use a limited voltage supply as efficiently as possible, the full voltage swing should be used (positive and negative) for the output.
A technique that uses two output signals efficiently is linear amplification with nonlinear components (LINC), which is described in the literature, including U.S. Pat. No. 4,178,557 to Henry and U.S. Pat. No. 7,260,368 to Blumer, and X. Zhang and E. Larson, “Gain and Phase Error-Free LINC Transmitter”, IEEE Trans. Vehicular Tech. Vol. 49, No. 5, pp. 1986-1994 (September 2000). In LINC, two equal signals (using separate power amplifiers) are phase-shifted in relation to each other. When the signals are perfectly in phase, the output signal is zero, and as the relative phase shift increases, the amplitude of the output signal increases until the signals are 180-degrees out of phase. For a Class D amplifier, the signals are two square waves.
As illustrated by FIG. 3, LINC is not strictly speaking a pulse-width modulation technique because the pulse-widths of the output signals A, B of the LINC arrangement are constant. Nevertheless, the combination (difference) signal A-B can be considered pulse-width modulated in that the widths of the pulses in the difference signal depend on the envelope component of the signal input to the LINC arrangement and the temporal positions of the pulses depend on the phase component of the signal input. A problem with LINC is that there is no low-frequency information in the individual outputs A, B to be used in a feedback loop for linearization. Overcoming that problem is complicated and uses devices that consume substantial power.